The present invention relates to a flexible shaft coupling and a method of manufacturing the same, and in particular to a flexible shaft coupling using a rubber member or an elastomer member for an intermediate element thereof and a method of manufacturing the same.
Some of the known flexible couplings comprise a first shaft coupling member fixed to an end of a first shaft member, a second shaft coupling member fixed to an end of a second shaft member and an elastomer member (elastic rubber member) serving as an intermediate member connecting the first and second shaft coupling members to each other. See JPS48-25939Y, JPH10-331863A, JP2004-286181A, JP4315453B and JP4621728B.
In connection with such flexible shaft couplings, it has been proposed to improve a torque transmission efficiency, a capability to withstand repeated compression and a capability to absorb vibrations by appropriately selecting the modulus of repulsion elasticity and the rubber hardness of the elastomer member (see JP 10-331863A, for instance), to improve responsiveness by increasing the torsional spring constant of the elastomer member in the direction of rotation (circumferential direction) to an appropriate level by suitably selecting the thickness of the elastomer member (see JP4315453B, for instance), and to allow the vibrations in the axial direction to be buffered by extending the elastomer member in the axial direction (thrust direction) up to the end surface of the coupling member (see JP2004-286181A, for instance).
Even when a first shaft member and a second shaft member which are to be coupled with each other are radially offset from each other, when the axial lines of the first and second shaft members are at an angle to each other, and/or when the axial distance between the opposing ends of the first and second shaft members is varied, a flexible shaft coupling can accommodate such a deviation by means of the elastic deformation of an elastomer member.
For the elastomer member to be able to accommodate positional deviations such as radial offset, angular offset and axial offset between the first and second shaft members by means of elastic deformation thereof, it is preferable that the elastomer member is made of soft rubber material having a low rubber hardness. On the other hand, the softer the elastomeric material is, the greater will be the radial elastic deformation of the elastomer member when transmitting a given torque between the first and second shaft members.
Therefore, when a soft rubber material is used for the elastomer member with the aim of accommodating the positional deviation between the first and second shaft members, not only the rotational angular difference between the first and second shaft members may increase to an unacceptable level but also the durability of the elastomer member may be reduced and the service life of the shaft coupling may be reduced.
If the elastomer member is made of a hard rubber material, the rotational angular difference between the first and second shaft members may not increase to an unacceptable level, and the durability of the elastomer member may not be reduced. However, when there is a significant positional deviation between the first and second shaft members as is often the case where a flexible shaft coupling is required, the positional deviation causes a persistent stress to the elastomer member in addition to the stress caused by the normal transmission of torque.
In particular, the stress owing to the positional deviation between the two shaft coupling parts may be produced in highly localized parts of the elastomer member, and this could cause an unpredictable fatigue or other damages to the elastomer member. Also, the use of a hard rubber material for the elastomer member may reduce the capability of the shaft coupling to accommodate the positional deviation between the first and shaft members.
It is also conceivable to place the elastomer material only in parts of the flexible shaft coupling where the elastomer material is loaded solely by the torque that is to be transmitted, and not by the stresses caused by the angular and other positional deviations between the first and second shaft coupling members of the flexible shaft coupling. For instance, each shaft coupling member may be provided with a plurality of claws arranged circumferentially at a regular angular interval and projecting toward the other shaft coupling member such that the claws of the two shaft coupling members interdigitate one another. If the elastomer material is filled only into the circumferential gaps between the adjacent claws, the elastomer material fully receives the loading caused by the torque transmission but is not substantially subjected to loadings owing to the positional deviations between the two shaft coupling members. However, the molding die that would be required for manufacturing such a flexible shaft coupling is so complex that a relatively high manufacturing cost would be required.